Sewage treatment

ABSTRACT

The sewage treatment process combines physico-chemical clarification using fine material or clay particles with anaerobic digestion of a concentrated sewage. The particles are preferably magnetite. The sewage (10) is mixed with magnetite (12) having a hydroxylated surface layer in tanks (14, 16, 18, 20) in which acid, inorganic flocculant and polyelectrolyte may be added if necessary. The organic material in the sewage is adsorbed on the particles during the mixing contact and a clarified liquid effluent (28) is separated from the loaded particles in a clarifier (22). The organic material may be separated from the particles before or after treatment in an anaerobic digestion assembly (36). The particles are preferably cleansed and regenerated in a two stage countercurrent flow washing system (38, 40, 58, 60, 62) into which a dilute solution of caustic soda or lime (32) is introduced.

This invention relates to a new process for sewage treatment. Theprocess of the invention combines physico-chemical clarification usingfine mineral or clay particles with anaerobic digestion of aconcentrated sewage.

Sewage can be regarded as the water-borne waste products of man. Eversince man has gathered himself into large communities, the effectivetreatment and disposal of sewage has been a matter for concern.Initially, the problem was solved by discharge into tidal waters or aninland watercourse. However, with an increase in population density,this option became unworkable and some form of treatment becamenecessary.

The main aim of sewage treatment is to greatly reduce both thebiological oxygen demand (BOD) of the sewage and the number ofpathogenic organisms. Recently, the removal of inorganic nutrients(phosphorus and nitrogen) has also become important. Historically, bothclarification in settling tanks and biological oxidation have been used.Biological oxidation can be achieved in lagoons, trickling filters oractivated sludge plants, the particular process chosen depending on suchfactors as land availability, sewage strength and power costs. In anindustrialized economy, the activated sludge process is usually selectedbecause of smaller land requirements and the relative cheapness ofpower.

Up to date capital and operating costs for an activated sludge plant aredifficult to obtain, but some idea of costs can be gained by takinghistorical costs and updating to the present. Analysis of costinformation for the years 1968 to 1977 shows that, when indexed to 1986,a direct operating cost of approx. 10¢/m³ is obtained, while the capitalcost of a 38 ML/day plant is estimated to be A$16×10⁶. Amortization ofcapital cost over 25 years at 15% p.a. results in a contribution tototal treatment cost of approx. 20¢/m³. Total treatment cost of thesewage is thus of the order of 30¢/m³.

With industrialization, sewage flows and, as a consequence, total costshave increased dramatically. While a large fraction of these costs canbe attributed to the collection system (drains, sewers etc.), treatmentcosts are still a significant fraction of the total, especially whenmore stringent effluent standards are enforced. As a consequence, thereis considerable incentive to reduce treatment costs, and recent effortsto apply anaerobic techniques to sewage treatment have had this aim.

The invention seeks to provide a process for the treatment of domesticsewage, which will significantly reduce total treatment costs. Whileconventional processes generally rely on aerobic biological oxidation,this process proposes to use a combination of efficient physico-chemicalclarification with rapid anaerobic biological digestion. The reductionin treatment costs can be achieved by greatly reducing capital costs,while limiting operating costs to below those for the conventionalprocess. As capital costs represent about two thirds of the totaltreatment cost, a significant cost reduction should result.

The first stage of the present process involves physico-chemicalclarification of raw sewage or primary settled sewage. This part of theprocess derives from the so-called "Sirofloc" process for waterclarification, various aspects of which are described in AustralianPatents Nos. 512,553, 518,159 and 550,702, the full texts of which arehereby incorporated into the present specification. Basically, theSirofloc process provides rapid and efficient removal of suspendedimpurities and coloured substances (turbidity and colour colloids) fromwater by treatment of the water with a finely divided particulatemineral or clay material (referred to as a "coagulant/adsorbent") theindividual particles of which have a particle size of 10 microns or lessand have a thin hydroxylated surface layer. A positive zeta potential onthe surface of the coagulant/adsorbent particles is not considerednecessary for the present process.

Operation of the Sirofloc process is often enhanced by the addition tothe water under treatment of a suitable coagulant, such as alum orferric chloride, and/or a polyelectrolyte (cationic, anionic ornon-ionic). The preferred coagulant-adsorbent in the process ismagnetite. Apart from the ease with which the required hydroxylatedsurface layer can be formed (and regenerated) on magnetite, the magneticproperties of the mineral can be usefully employed to aid in separationof the coagulant/adsorbent from the treated water.

We have found that there is a strong tendency for sewage organics toco-flocculate with coagulant/adsorbents such as magnetite. The rapidadsorption and flocculation with such materials contrasts strongly withthe long contact times and slow clarification of the activated sludgeprocess.

We have also found surprisingly, that the coagulant/adsorbents canadsorb a significant fraction of the soluble chemical oxygen demand(COD) present in sewage as well as removing suspended and colloidalmaterial. The ability of the coagulant/adsorbents, and particularlymagnetite, to remove soluble organic material is contrary toexpectations from the studies that have been made of the Siroflocprocess, and it enables a high degree of clarification of the sewage tobe achieved.

Anaerobic digestion of concentrated sewage sludges has long beenpractised as a means of stabilizing the sludge prior to disposal, and inrecent years there has been a strong resurgence of interest in theanaerobic treatment process. However, although raw sewage can be treatedby anaerobic digestion the quality of the treated sewage is frequentlyunacceptable. The anaerobic digestion of raw sewage at ambienttemperature is slow, especially in cold climates where the process maybe inoperable. Moreover, the large volume of dilute material to betreated generally makes it impractical or uneconomic to carry outanaerobic digestion of raw sewage at elevated temperatures in order toaccelerate the digestion and reduce the residence time required. Forexample, insufficient methane is produced in the anaerobic fermentationof raw sewage to serve as fuel for heating the large volume of dilutematerial, and the use of supplementary fuels is generally prohibitivelyexpense.

The present invention overcomes these limitations by employingphysicochemical clarification to provide an effluent of high qualitywhile simultaneously concentrating the organic material in the sewageinto a sludge suitable for rapid anaerobic digestion.

According to the present invention, there is provided a sewage treatmentprocess which comprises the steps of:

(a) mixing raw sewage or primary settled sewage with acoagulant/adsorbent which is a finely divided particulate mineral orclay material the individual particles of which have a thin hydroxylatedsurface layer, under conditions whereby at least a substantialproportion of the organic material in the sewage becomes attached to thecoagulant/adsorbent;

separating the coagulant/adsorbent with attached organic material fromthe mixture to leave a treated liquid effluent; and

(c) subjecting the thus separated organic material to anaerobicdigestion.

As will appear from the following discussion, step (c) may be carriedout while the organic material is still attached to thecoagulant/adsorbent; or preferably, the organic material may be removedfrom the coagulant/adsorbent before anaerobic digestion.

The anaerobic digestion may be performed in any known type of digestersuitable for mesophilic or thermophilic digestion.

The coagulant/adsorbents which may be used in accordance with thepresent invention may be of two notionally different types, i.e.: (I)those in which the hydroxylated layer is derived directly from thesubstance of the particles; and (II) those in which the layer is derivedfrom another substance.

The preferred coagulant/adsorbent materials are those of type I andthese can be derived from a wide variety of minerals and clays providedthe nature of the mineral is such as to permit the ready formation ofthe hydroxylated surface. In this respect oxides and silicates areparticularly useful. Examples of such minerals include zinc oxide,silica and siliceous materials such as sand and glass and clay mineralssuch as mica, china clay and pyrophillite. This list is not exhaustive,however, and many other minerals are suitable for use in this invention.

In the most preferred embodiment of this invention, the particulatematerial is a magnetic or magnetisable material. For this purpose ironoxides, such as gamma iron oxide or magnetite, which are eminentlysuitable, or ferrites, such as barium ferrite or spinel ferrite, can beused.

The coagulent/adsorbent particles should have a particle size of 20microns or less, preferably 1 to 10 microns. It is believed that theoptimum size range for the preferred method of operating the process is1 to 5 microns.

The preparation of finely divided coagulant/adsorbent particles of typeI to give each a thin hydroxylated surface layer is easily carried out,usually by suspending the particles in a basic, preferably an alkali,solution for a short period of time, preferably in the presence of air.Sodium hydroxide is suitable, but potassium hydroxide, lime or aqueousammonia may also be used. Generally, alkali concentrations should be atleast 0.01N, preferably about 0.05N to 0.1N, at which level thetreatment is effective after about 10 minutes. Shorter treatment timescan be achieved by the use of elevated temperatures and/or higher alkaliconcentrations. A suggested temperature range is 40°-60° C. For example,a satisfactory material is produced using either 0.1N sodium hydroxideat room temperature (i.e. about 20° C.) for ten minutes, or 0.05N sodiumhydroxide solution at about 60° C. for five minutes.

Because the hydroxylated layer of the type II coagulant/adsorbent isprovided by a different substance, to the material of the mineral orclay particle the range of starting materials is broader. A wide varietyof minerals and clays can be used provided the nature of the mineral orclay is such as to permit the ready deposition of a hydroxide gel on itssurface. In this respect oxides, sulphates, silicates and carbonates areparticularly useful. Examples of such minerals include calcium sulphate,calcium carbonate, zinc oxide, barium sulphate, silica and siliceousmaterials such as sand and glass and clay minerals such as mica, chinaclay and pyrophillite. This list is not exhaustive, however, and manyother minerals are suitable for use in this invention. In some cases,pre-treatment of the surface of the mineral may be required to produce asatisfactory deposition of the hydroxide layer. Yet another alternativeis to use hollow microspheres, e.g. of glass for the production of gelparticles which can be separated from the liquid effluent, after theadsorbtion of the organic material in step (a), by flotation rather thansedimentation.

The hydroxylated layer of the coagulant/adsorbent particles of type IIcan be provided by any of a number of metal hydroxides, the requirementsbeing substantial insolubility in water and a valency preferably ofthree or more.

Suitable metals with this characteristic are ferric iron, aluminium,zirconium and thorium. Ferric hydroxide is preferred because it ischeap, and exceptionally insoluble, over a wide pH range. For example,it does not readily dissolve at high pH, as does aluminium hydroxide.

The preparation of the coated particle of type II is also easily carriedout, usually by suspending the particles in water, adding a salt of asuitable metal followed by an alkaline material, preferably in aqueoussolution which will precipitate the metal hydroxide which then forms acoating on the particle. Typically, chlorides, sulphates, nitrates orother mineral acid salts of the metals are suitable; ferric chloride oraluminium sulphate are examples. The alkaline material may be sodiumhydroxide, calcium hydroxide, ammonia or similar soluble material. Theconcentration and temperature at which the preparation is carried out isgenerally not critical.

In the case where magnetite or other iron oxide materials are used asthe basis for type II particles, the metal salt which is employed toproduce the hydroxide layer may be obtained by first adding acid to thesuspension of the particles (to give ferric and/or ferrous salts insolution from the iron oxide) and then adding the alkaline material.

It has been found advantageous, when forming the particles of type II toprovide means for increasing the degree of polymerization of thehydroxide layer. Polymerization occurs due to elimination of water andthe establishment of oxygen ("ol") linkages between the metal atoms:

    2MOH$MOM+H.sub.2 O

This process occurs on standing, but can be accelerated by heating.

After preparation, it is best if the coated particles are not permittedto dry out. This can be avoided by keeping them under water. Thethickness of the hydroxylated layer on the particles is not importantsince the flocculation or coagulation is a surface effect.

An important advantage of the process of the present invention is thatthe coagulant/adsorbent particles can be recycled many times. To achievethis, the adsorbed material is removed by raising the pH of a suspensionof the adsorbent in water. In the case of type I coagulant/adsorbents,the coagulating properties may be regenerated by treatment with alkalisolution; these two treatments may be combined.

As in the Sirofloc process the process of the present invention may beenchanced by the addition to the liquid under treatment of a suitablecoagulant, such as polyelectrolyte (cationic, anionic or non-ionic)and/or an inorganic coagulant which provides multi-valent cations suchas Fe²⁺ (e.g. ferrous sulphate). More usually the multi-valent cationswill have a valency of three or more, such as Fe³⁺ or Al³⁺, (e.g. fromalum or ferric chloride). These coagulants are not essential but whenboth types (i.e. polyelectrolytes and the inorganic coagulants) arepresent they complement each other. The polyelectrolyte may be presentin the range 0 to 10 mg/L, preferably from 2 to 5 mg/L. The inorganiccoagulant may be present in the range 0 to 100 mg/L, preferably 20 to 50mg/L.

The preferred coagulant/adsorbent is magnetite and the followingdetailed description will refer to that material. It will be appreciatedhowever, that reference to magnetite includes mutatis mutandis referenceto other coagulant/adsorbents.

Reference will now be made to the accompanying drawings in which:

FIG. 1 is a flow diagram showing the process of the invention in itsbasic and alternative forms;

FIGS. 2 and 3 are graphs showing performance details of the process inits alternative form, as described in the following examples;

FIG. 4 is a detailed flow diagram of a preferred embodiment of theprocess of the invention; and

FIG. 5 illustrates a preferred anaerobic digester layout.

The basic process of the invention is shown by the solid connectinglines in FIG. 1.

Raw or primary settled sewage is mixed with finely-divided cleanedrecycled magnetite particles which have been regenerated by suspensionin a solution of caustic soda to produce a thin hydroxylated surfacelayer, and the mixture is stirred to provide good contact of sewage withthe regenerated magnetite particles which are preferably in the sizerange 1 to 10 microns. The pH level may be adjusted by acid addition tobe in the range 5 to 9, preferably 5.5 to 6.5 and the addition of aninorganic coagulant and/or a coagulant aid (e.g. a polyelectrolyte) mayalso be necessary to achieve a satisfactory effluent quality dependingon the strength and composition of the raw sewage. After 2 to 20minutes, preferably 10 to 15 minutes, of contact the magnetite slurry isseparated from the treated effluent which may go to polishing ponds forfinal treatment before discharge. The magnetite particles, which nowhave attached to them most of the organic material originally present inthe sewage, can be passed to an anaerobic digester where, because of thehigh concentration of organic material, rapid digestion at an elevatedtemperature of 35° to 40° C. may be achieved. Digestion may be performedat a temperature in the range of about ambient to about 70° C. buttemperatures below about 30° C. result in insufficiently rapid digestionwhile temperatures between 40° C. and about 70° C. may speed up theprocess but would require additional heat. After exit from the digester,the magnetite may be cleaned by stripping of the digested sludge using adilute solution of sodium hydroxide, ammonia, or potassium hydroxide or,for example, a lime slurry which also regenerates the magnetite. Themagnetite can then be recycled while the stripped sludge is sent todrying beds for disposal.

A successful sewage treatment process based on the above procedureoffers great scope for a reduction in capital cost as the residence timeof the main sewage flow in the plant would be no longer than 30 minutescompared with approximately 8 hours for an activated sludge plant. Theflow through the anaerobic digestion stage would be only about 1-2% ofthe main sewage flow and, assuming a high rate of digestion can beachieved, the size of the digester should be smaller than the sludgedigester associated with a conventional activated sludge plant.

In the alternative, and more preferable process shown by the dashedlines in FIG. 1, the organic material is stripped from the magnetitebefore anaerobic digestion. The magnetite is recovered, regenerated andrecycled as before.

Various aspects of the process of the invention are further describedand discussed hereinafter. This material should not be construed aslimiting on the nature or scope of the invention.

CLARIFICATION

There is a variety of parameters which can affect the efficiency of theclarification process. However, once the mixing and settling conditionshave been prescribed, it is the addition of chemicals (including thefine hydroxylated magnetite particles) which controls the effluentquality. The preferred dose of hydroxylated magnetite on a dry weightbasis is in the range 5 to 40 g/L of sewage. Jar tests were used for apreliminary study of the clarification process, and a sample of resultsis given in Table 1. It was possible to obtain a high quality effluent,similar to that from an activated sludge plant, simply by increasing thedose of chemicals; e.g. at a chemical cost of 10¢/m³ an effluent with aBOD of 25 mg/L and suspended solids of less than 1 mg/L was achieved,while at a more reasonable chemical cost of 5¢/m³ effluent BOD andsuspended solids levels were 38 and 3.4 mg/L respectively. The removallevels of COD in Table 1 vary from 70 to 90% and compare favourably withthose reported for a full anaerobic treatment of raw sewage.

                                      TABLE 1                                     __________________________________________________________________________    Clarification of Raw Sewage with Fine Magnetite Particles                     Raw sewage characteristics"                                                   SS 78 mg/L                                                                    COD 500 mg/L                                                                  BOD 220 mg/L                                                                                      Treated Sewage                                                                Suspended                                                                             Quality                                                          Chemical                                                                           Solids  COD BOD                                           Treatment Conditions                                                                         Cost (mg/L)  (mg/L)                                                                            (mg/L)                                        __________________________________________________________________________      FeCl.sub.3 - 20 mg/L                                                                        4¢/m.sup.3                                                                   8       150 43                                              Polyelectrolyte - 4 mg/L                                                      Acid - 0.5 mmol/L                                                             Magnetite - 10 g/L                                                            FeCl.sub.3 - 20 mg/L                                                                        5¢/m.sup.3                                                                   3.4     120 38                                              Polyelectrolyte - 4 mg/L                                                      Acid - 1 mmol/L                                                               Magnetite - 20 g/L                                                            FeCl.sub.3 - 50 mg/L                                                                       10¢/m.sup.3                                                                   0.9      44 25                                              Polyelectrolyte - 5 mg/L                                                      Acid - 1 mmol/L                                                               Magnetite - 10 g/L                                                          __________________________________________________________________________

ANAEROBIC DIGESTION

The principal advantage of the present process, lies in the ability toachieve concentration of the organic material in raw sewage to a levelthat allows the organic material to be rapidly digested anaerobically.This concentrated material can be fed to the digester either attached tothe magnetite particles used in clarification or as a concentratedslurry after stripping and separation from the magnetite particles. Ineither case the material is concentrated by a factor of about 60 overthe raw sewage. Measured COD levels on a stripped slurry average around30,000 mg/L, an ideal level for feeding to a mesophilic or thermophilicanaerobic digester. Anaerobic digestion of concentrated wastes is now acommercially successful process with a variety of digester designs beingavailable.

DIGESTER OPERATION

In initial investigations of anaerobic digestion the concentratedmagnetite slurry as separated from the liquid effluent was used as thefeed material. In this situation the sewage material remains attached tothe fine magnetite particles having a particle size in the range 1 to 10microns, and it was hoped that in the digester these particles would berapidly colonized by anaerobic bacteria, ensuring close proximity offood and microorganisms. The magnetite slurry had a water content ofabout 90% v/v (sp.gr. 1.4).

However, the results from four weeks of digester operation at pH 7 and atemperature of 35° C., with seed material from a conventional anaerobicsludge digester, were disappointing. At a feed rate of 5 kg COD/m³ d,gas analyses appeared satisfactory, the process taking about 2 weeks toreach a steady state with methane and carbon dioxide reaching 55 and 25%respectively on a w/w basis. Hydrogen concentrations reachedundetectable levels after 2 weeks of operation. However, gas productionrates were about 0.12 m³ /m³ d, an order of magnitude below what wouldbe expected from a high rate digester, and showed no signs ofincreasing. A variety of reasons can be put forward to explain this poorresult. The anaerobic bacteria may have been unable to attach themselvesto the fine magnetite particles (1-10 mm), which are of a comparablesize. As a consequence, access to their food supply may have beenlimited and this might have been alleviated by increasing the maximumparticle size to 20 mm. Possible nutrient deficiencies, especially inphosphate which can be adsorbed by the magnetite particles, could alsohave caused or contributed to the poor result. The limited residencetime of the magnetite particles in the digester may not have allowedsufficient bacterial colonization to occur.

Whatever the reason for the failure of the initial approach, improveddigestion can be achieved by the alternative procedure of stripping andseparating the concentrated sewage material from the magnetite particlesand then feeding the resultant slurry to an anaerobic digester. Theregenerated magnetite particles can then be directly recycled for afurther clarification step.

In this approach, the anaerobic digester advantageously, but notessentially, contains larger magnetite particles, for example of size50-100 mm to serve as the site for bacterial attachment and growth andthese particles are permanently retained within the digester by recyclee.g. from an attached settling cone. The anaerobic digester could be ofany other type suitable for mesophilic or thermophilic digestion but thepresence of the larger magnetite particles is advantageous because theyremove H₂ S from the reactor environment.

The results from the first sixty days of operation of such a digester,with a concentrated regeneration effluent as feed, are given in FIGS. 2and 3. For the first week the COD of the feed was around 14,000 mg/L,but this level was lowered to between 9 and 10,000 mg/L after the firstweek by the addition of a more dilute washing effluent. Nominal liquidresidence time was 2.5 days. The digester was seeded from materialsupplied by Bunge Pty. Ltd. During the first ten days of operationbetween 80 and 100% removal of COD was obtained, with an initial gasevolution rate of 0.4 m³ /m³.d and a methane content of around 50%.Unfortunately, during the first week an undetected blockage in the gassampling line prevented further measurements of gas rates during thisperiod. However, it is probable that the high COD removal rates observedduring this period were the result of a combination of physicaladsorption and microbiological digestion. After day 10, although gasevolution rates of up to 0.8 m³ /m³.d were initially observed, CODremoval fell to about 40% and remained at this level for the duration ofthe operation. Gas evolution rates slowly declined although methanecontent never fell below 50% and increased up to 70% after day 50.

FIG. 4 is a process flow diagram of the presently preferred overallprocess which essentially follows the alternative process of FIG. 1. Theraw sewage at 10 is mixed with cleaned, recycled and regeneratedmagnetite particles at 12 at a rate of 5 to 40 grams dry weight perlitre of raw sewage (preferably 10 to 20 g/L) and the aqueous mixture isstirred to provide good conatct between the sewage and the hydroxylatedmagnetite particles. This stirred contact can be achieved over a periodof 2 to 20 minutes, preferably 10 to 15 minutes, in a series of fourstirred tanks 14, 16, 18 and 20 as shown with acid being added in tank14 to adjust the pH to within the range 5 to 9, preferably 5.5 to 6.5,inorganic flocculant such as alum being added in tank 16 andpolyelectrolyte being added in tank 18. The quantities of the latter twocomponents added will depend upon the quality of the treated sewagerequired as well as the strength and composition of the raw sewage butthese two components may not be required at all. After the final mixingtank 18 the sewage/magnetite mix passes to a clarifier 22 on the way towhich the magnetite is magnetized by a flocculating magnet 24, which isplaced around the entrance pipe 26 to the clarifier 22. The clarifiermay be of any suitable known type, but advantageously incorporates theimprovement described in our Australian Patent specification No. 553,423the contents of which are incorporated herein by reference. In theclarifier the loaded magnetite particles rapidly separate from theclarified sewage and this liquid effluent is exhausted from the processat 28. The loaded magnetite particles are extracted from the clarifier22 at 30 and must then be regenerated, for example, by mixing them witha dilute solution of caustic soda which is introduced to the system at32. Alternatively a lime slurry may be added. This regeneration processcan produce a liquid stream 34 of sewage concentrate with a chemicaloxygen demand (C.O.D.) in the range 10,000-20,000 mg/L, which is in theideal range for feeding to a mesophilic anaerobic digester assembly 36.

As shown in FIG. 4 the preferred regeneration process involves a twostage countercurrent flow washing operation, which uses magnetic drumseparators 38 and 40, which may be of known type, to separate themagnetite particles from recycled wash water in the two stages. Theliquid effluent 42 from the first magnetic drum separator 38 passes tothe anaerobic digester assembly 36, where the sewage organics are brokendown to form methane which is exhausted at 44. The effluent 46 from thedigester assembly 36 passes to a sludge settling pond 48 whereparticulate biomass settles out and may be disposed of as a dried sludgeat 50. The overflow 52 from the pond 48 is relatively clear andcolourless and has been found to have C.O.D. of less than 20% of that ofthe feed to the anaerobic digester assembly. Experiments have shown thatthis liquid overflow is suitable for recycling as the wash water 54. Thecaustic soda (NaOH) added to the waste water 54 at 32 raises the pHlevel to greater than 10 for regeneration of the magnetite and this alsoresults in ammonia (which is generated in the anaerobic digesterassembly 36) being stripped from the recycle stream by aeration at 56.This step alleviates the build up of ammonia in the wash water and thusallows the wash water to be recycled indefinitely. It will also preventany ammonia, in excess of that coming in with the raw sewage, fromfinding its way into the clarified sewage effluent. Thus, the onlyliquid or solid effluents from the process will be the clarified sewage28 and the dried anaerobically stabilised sludge 50.

The recycled wash water 54 with ammonia removed is mixed with partlycleansed and regenerated magnetite in a stirring tank 58 in themagnetite flow line between the magnetic drum separators 38 and 40. Theoverflow from the tank 58 flows to the separator 40 where the fullycleansed and regenerated magnetite with hydroxide layer is extracted forrecycle. The separator wash water is directed along flow line 60 into astirring tank 62 in the magnetite flow line between the clarifier 22 andthe magnetite drum separator 38. The recycle water and the loadedmagnetite removed from the clarifier 22 are thoroughly mixed in the tank62 and the overflow passes into the magnetic drum separator 38. Aspreviously described the liquid effluent from the separator 38 passes tothe anaerobic digester assembly 36 which may be as shown in FIG. 5.

The presently favoured technique for operation of the anaerobic digesterassembly is to strip the sewage material from the magnetite particleswith a dilute caustic soda solution to produce a sewage concentrate,which is then fed to an anaerobic digester assembly 36 as describedabove. To date, the best digester results have been obtained with anupflow anaerobic sludge blanket digester 64, operating at a laboratoryscale in a 10 cm diameter glass column 66. This digester is preceded bya 5.5 L stirred pot 68, incorporating a magnetic stirrer 70, in whichacidification of the sewage concentrate supplied from a feed tank 72takes place at a pH of about 5.7. The anaerobic sludge blanket digester64 was operated at a pH of about 7.2. The sewage concentrate had a pHlevel of about 10, but the acidification reactor 68 was still able tomaintain its pH level with this feed. Some typical results of thedigester operation are given in Table 2.

                  TABLE 2                                                         ______________________________________                                                               Loading Rate                                                                  g COD/g    Hydraulic                                   C.O.D.       % removal dry wt     Retention                                   mg/L         of C.O.D. biomass · d                                                                     Time                                        ______________________________________                                        Feed    6,800    --        --                                                 Acidifiction                                                                          5,850    14%       0.11     6.1 days                                  Reactor (at exit)                                                             Anaerobic                                                                             1,200    82%       0.08     2.2 days                                  Sludge  (at exit)                                                             Blanket                                                                       Digester                                                                      ______________________________________                                         Gas Production rate: 0.7 m.sup.3 /m.sup.3 · d                        Table 1: Digester Operating Data                                         

The results show a very good removal (82%) of C.O.D. through thedigester system at quite reasonable loading rates. The effluent from thesludge blanket digester 64, after clarification in a settling pond (notshown), is sufficiently clear to allow the recycle and reuse of thismaterial as wash water in the magnetite regeneration process as shown inFIG. 4. 75% of the gas given off at 24 from the digester was methane and20% carbon dioxide.

The results obtained clearly demonstrate that the sewage concentrateobtained by stripping the magnetic particles can be rapidly digestedanaerobically.

We claim:
 1. A sewage treatment process characterized in that it comprises the steps of:(a) mixing raw sewage or primary settled sewage with a coagulant/adsorbent which is finely divided particulate mineral or clay material the individual particles of which have a thin hydroxylated surface layer, under conditions whereby at least a substantial proportion of the organic material in the sewage becomes attached to the coagulant/adsorbent; (b) separating the coagulant/adsorbent with attached organic material from the mixture to leave a treated liquid effluent; and (c) subjecting the thus separated organic material to anaerobic digestion.
 2. A process according to claim 1 wherein the coagulant/adsorbent particles are of magnetite.
 3. A process according to claim 1 wherein each coagulant/adsorbent particle has a particle size of 20 microns or less.
 4. A process according to claim 1 wherein the coagulant/adsorbent particles are regenerated from a previous cycle of the process, to provide them with the thin hydroxylated surface layer.
 5. A process according to claim 1 wherein the coagulant/adsorbent particles are added in an amount in the range of 5 to 40 grams dry weight per litre of sewage.
 6. A process according to claim 1 wherein an additional inorganic coagulant providing multivalent cations is mixed with the sewage and coagulant/adsorbent in an amount of 0 to 100 mg/L of sewage.
 7. A process according to claim 6 wherein the multivalent cations are selected from Fe²⁺, Fe³⁺ and/or Al³⁺.
 8. A process according to claim 1 wherein a polyelectrolyte is mixed with the sewage and coagulant/adsorbent in an amount of 0 to 10 mg/L of sewage.
 9. A process according to claim 1 wherein the pH of the sewage and coagulant/adsorbent mixture is adjusted to in the range of 5 to
 9. 10. A process according to claim 1 wherein the contact time for the sewage and coagulant/adsorbent mixture prior to step (b) is in the range 2 to 20 minutes.
 11. A process according to claim 1 wherein the coagulant/adsorbent particles are of magnetic material and the particles are magnetised between steps (a) and (b).
 12. A process according to claim 1 wherein the organic material is subjected to anaerobic digestion attached to the coagulant/adsorbent.
 13. A process according to claim 1 wherein the organic material is separated from the coagulant/adsorbent prior to anaerobic digestion.
 14. A process according to claim 13 wherein the coagulant/adsorbent particles are cleansed and regenerated by a two-stage separating and backwashing treatment in which the pH is increased to at least 10 and the organic material is separated from the particles in a first separator in the particle flow path.
 15. A process according to claim 1 wherein the coagulant/adsorbent particles are separated from the organic material using a dilute solution of sodium hydroxide, potassium hydroxide or ammonia or using a lime slurry.
 16. A process according to claim 1 wherein the anaerobic digestion is performed at a temperature in the range 30° to 40° C.
 17. A process according to claim 1 wherein the anaerobic digestion is performed in an upflow anaerobic sludge blanket digester. 